Quantum Control of Tightly Competitive Product Channels

Fundamental selectivity limits of quantum control are pushed by introducing laser driven optimal dynamic discrimination to create distinguishing excitations on two nearly identical flavin molecules. Even with modest spectral resources, significant specificity is achieved with optimal pulse shapes, which amplify small molecular differences to create distinct, identifying signals. Rather than being a hindrance, system complexity appears to aid the control process and augments control field capability, which bodes well for implementation of quantum control in a variety of demanding applications.

Figure 1

RBF and FMN are structurally (a) and spectroscopically (b) quite similar. A shaped UV pulse coordinated with a time-delayed IR pulse enables selectivity (c) and is able to both maximize and minimize the fluorescence depletion ratio by $\pm 28\%$ (d). Arrows at 400 nm and 530 nm in (b) locate regions of UV control and collected fluorescence, respectively.

Figure 2

Absolute RBF and FMN depletion signals for optimized UV pulse shapes at $\tau =500\mathrm{fs}$. Absolute depletions induced by the TL pulse for both RBF (black) and FMN (gray) are statistically equivalent at $\sim 26\%$. Optimal pulses pull apart the RBF (blue) and FMN (gold) distributions to produce the corresponding depletion ratios shown in Fig. 1d.

Figure 3

Flavin ODD as delay $\tau$ is varied between the unshaped IR component and the optimized UV component (optimized at $\tau =500\mathrm{fs}$). Optimal pulses provide good discrimination out to $\tau \sim 1\mathrm{ps}$. The inset displays the transform-limited pulse and richly structured optimal UV fields. Field colors correspond to depletion signals.

Figure 4

Distinct optimal UV pulses that obtain the same degree of discrimination. The short-time Fourier transforms reveal multiple subpulses that commonly show a pattern of frequency downchirp followed by an upchirp for minimization of $D_{\mathrm{FMN}}/D_{\mathrm{RBF}}$ (a) while the solutions for maximization of $D_{\mathrm{FMN}}/D_{\mathrm{RBF}}$ (b) display the opposite chirp pattern. Subtle structure in the UV pulses is crucial, as attempts at creating simplified pulse forms following the observed chirp pattern failed to provide quality discrimination.